In a typical exposure apparatus of this kind, a light beam emitted from a light source enters a fly's eye lens. Then, a secondary light source constructed from many light sources is formed on the rear side focal surface thereof. A light beam from the secondary light source is limited through an aperture stop arranged in the vicinity of the rear side focal surface of the fly's eye lens, and then, enters a condenser lens. The aperture stop limits a shape or a size of the secondary light source to a desired shape or a desired size according to desired illumination conditions (exposure conditions).
A light beam condensed by the condenser lens illuminates in a state of superimposition a mask in which a given pattern is formed. Light passing through the pattern of the mask forms an image on a wafer through a projection optical system. Thereby, the mask pattern is projected and exposed (transferred) on the wafer. The pattern formed on the mask is highly integrated. Therefore, in order to correctly transfer this fine pattern on the wafer, it is essential to obtain a uniform illumination intensity distribution on the wafer.
In the exposure apparatus having the foregoing construction, it is necessary to set minute lens elements constructing the fly's eye lens as much as possible in order to improve uniformity of the illumination intensity distribution. Further, it is necessary to form the secondary light source in the shape close to a desired shape in order to avoid light amount loss in the aperture stop. Therefore, for example, it is thinkable that a size of the minute lens element constructing the fly's eye lens is set very small, that is, a micro fly's eye lens is used.
The fly's eye lens is constructed by arranging many lens elements vertically and horizontally and densely. Meanwhile, the micro fly's eye lens is constructed by integrally forming many minute refraction surfaces. That is, the fly's eye lens is constructed by combining and densely arranging many single polished lens elements. Meanwhile, the micro fly's eye lens is constructed by forming many minute refraction surfaces on, for example, a parallel plane glass plate by applying MEMS technique (lithography plus etching and the like).
Therefore, the fly's eye lens can be manufactured by checking refraction surface shapes of polished lens elements, selecting lens elements meeting standards, and using only lens elements having a high-precisely formed refraction surface. However, in the micro fly's eye lens, all minute refraction surfaces are required to be concurrently manufactured by etching through which is hard to obtain a quality surface shape compared to polishing. Therefore, a straight pass ratio thereof becomes considerably lower than that of the fly's eye lens.
In view of the foregoing problem, it is an object of the invention to provide an optical integrator having characteristics to reduce the effects of manufacturing errors of many minute refraction surfaces integrally formed by etching or the like on an illumination intensity distribution. Further, it is another object of the invention to provide a highly efficient illumination optical device capable of illuminating an irradiated surface under desired illumination conditions by using the optical integrator, wherein the effects of the manufacturing errors of the minute refraction surfaces on the illumination intensity distribution are reduced. Further, it is still another object of the invention to provide an exposure apparatus and an exposure method capable of performing good projection exposure under good illumination conditions by using the highly efficient illumination optical device capable of illuminating the irradiated surface under the desired illumination conditions.